The present invention relates to power conversion apparatus and methods, and more particularly, to power conversion apparatus including a DC bus and methods of operation thereof.
Uninterruptible power supplies (UPSs) and related power conversion apparatus often include a rectifier or other circuit that is used to generate a DC voltage at a DC output or link. For example, many UPSs use a configuration including an input rectifier circuit that receives an AC voltage from an AC power supply (e.g., an AC utility line) and that generates positive and negative DC voltages on a DC link including one or more busses (or “rails”). This DC voltage may be used to directly power loads (e.g., telecommunications equipment) or may be converted to AC by an inverter. In many such UPSs, an auxiliary DC power source, such as a battery coupled to the busses through a battery converter circuit, generates DC voltages on the busses in the event the AC power source fails and/or is degraded in quality.
A typical UPS includes one or more storage capacitors coupled to the DC link. Some on-line UPSs use a boost rectifier, which allows the DC voltage on the DC link to be boosted to levels greater than the peak AC input voltage. However, a potential problem With using such a boost rectifier in combination with a large storage capacitance is that undesirably large inrush currents may be produced when the rectifier is coupled to an AC source. In particular, in a boost mode converter, the DC link storage capacitors may initially act as a short-circuited load to the utility through the boost converter.
A precharge system may be used to pre-charge the DC bus to a voltage close to or preferably above the utility peak voltage prior to closure of utility disconnect device. Common approaches include using a power resistor that is coupled to the DC link with or without a series control device, such as a relay, to precharge the DC bus from the AC source. However, conventional precharge techniques may provide a long and/or difficult to scale pre-charge time or, conversely, may require the use of components that are bulky and expensive and may not be economical to assemble.